Stable somatic hybrids of mouse leukemia cells and other cell types lose their neoplastic qualities and are rejected by immunocompetent inbred mouse recipients of the same genotype as those used in forming the hybrid cells. The recipients who have rejected the leukemia cell hybrids exhibit partial specific immunity toward the leukemia cells used as one parent in forming the hybrid. Our project involves the use of hybrid cells in the prevention and treatment of passively transferred ASL-1 murine leukemia. We have established procedures to detect various parameters of humoral and cellular immunity toward ASL-1 cells and will monitor these parameters in mice rejecting hybrids of ASL-1 cells and various allogenic and xenogenic cell types. We will evaluate in syngeneic recipients the effect of hybrid cell immunotherapy on the survival of mice receiving passive transfer of viable leukemia cells.